Like a wristwatch that needs to be wound daily for accurate time-telling, the human circadian systemthe biological cycles that repeat approximately every 24 hoursrequires daily light exposure to the eye’s retina to remain synchronized with the solar day. Now Rensselaer researchers have demonstrated that when it comes to the circadian system, not all light exposure is created equal.

The findings have profound implications for exploring how lighting can be used to adjust our bodies’ clocks, and they could redefine the way lighting is manufactured, according to Mariana Figueiro, assistant professor in the Lighting Research Center (LRC).

Short-wavelength light, including natural light from the blue sky, is highly effective at stimulating the circadian system. Exposure to other wavelengthsand thus colorsof light may necessitate longer exposure times or require higher exposure levels to be as effective at “winding the watch.”

In some instances, exposure to multiple wavelengths (colors) of light simultaneously can result in less total stimulation to the circadian system than would result if either color were viewed separately, a phenomenon known as “spectral opponency.”

To demonstrate that the circadian system exhibited spectral opponency formed in the retina, the researchers exposed 10 subjects to three experimental conditions: one unit of blue light to the left eye plus one unit of green light to the right eye; one unit of blue light to the right eye plus one unit of green light to the left eye; and half a unit of blue light plus half a unit of green light to both eyes. They then measured each individual’s melatonin levels, a natural indicator of the circadian clock.

“Even though the amount of light was the same in all three conditions, when the two colors were combined in the same eye, the response of the system was reduced due to spectral opponent mechanisms formed in the retina,” says Figueiro.